1. Field of the Invention
The present invention relates generally to a mobile communication system using an HSDPA (High Speed Downlink Packet Access) service, and in particular, to a method for notifying all UEs (User Equipments) being provided with an HSDPA service of a change in HSDPA service information when the HSDPA service information is changed.
2. Description of the Related Art
In general, HSDPA refers to a technique for transmitting data including HS-DSCH (high-speed downlink shared channel) related control channels in a UMTS (Universal Mobile Telecommunication System) communication system. In order to support the HSDPA technique, an AMC (Adaptive Modulation and Coding) technique, a HARQ (Hybrid Automatic Retransmission Request) technique, and an FCS (Fast Cell Select) technique have been proposed. The AMC, HARQ and FCS techniques newly proposed to support the HSDPA technique will be described in detail herein below.
1) AMC Technique
The AMC technique is a data transmission technique for adaptively determining a modulation mode and a coding mode of different data channels according to a channel condition between a specific Node B and a UE (User Equipment), thereby increasing the overall utilization efficiency of the Node B. Therefore, the AMC technique has a plurality of modulation schemes and a plurality of coding schemes, and modulates and codes data channels by combining the modulation schemes and the coding schemes. Generally, each of combinations of the modulation schemes and the coding schemes are called “MCSs (Modulation and Coding Schemes)”, and there are a plurality of MCSs with level #1 to level #n according to the number of combinations of the modulation schemes and the coding schemes. In other words, the AMC technique adaptively determines a level of the MCS according to a channel condition between the Node B and the UE currently wirelessly connected to the Node B, thereby increasing the overall utilization efficiency of the Node B.
2) HARQ Technique
The HARQ, in particular n-channel SAW HARQ (n-channel Stop And Wait Hybrid Automatic Retransmission Request) technique, will now be described. For the HARQ technique, the following two plans to increase efficiency of an ARQ (Automatic Retransmission Request) technique have been proposed. In a first plan, the HARQ performs data retransmission request and response between the UE and the Node B. In a second plan, the HARQ temporarily stores errored data and then combines it with retransmission data of the corresponding data before transmission. In addition, the HSDPA technique has introduced the n-channel SAW HARQ technique in order to make up for the shortcomings of the conventional SAW ARQ (Stop And Wait ARQ) technique. The SAW ARQ technique transmits the next packet data only after receiving ACK (Acknowledgement) signal for the previous packet data. Therefore, in some cases, the SAW ARQ technique should await ACK, though it can presently transmit packet data. The n-channel SAW HARQ technique increases channel utilization efficiency by continuously transmitting a plurality of data packets before receiving the ACK for the previous packet data. That is, if it is possible to establish n logical channels between the UE and the Node B and identify the n channels according to time and channel numbers, the UE receiving the packet data can recognize the channel through which the received packet data was transmitted at a certain time, and take necessary measures by, for example, reassembling the received packet data in a desired order or soft-combining the corresponding packet data.
3) FCS Technique
The FCS technique rapidly selects a cell having a good channel condition among a plurality of cells, when a UE receiving an HSDPA service enters a cell-overlapped region, or a soft handover region. To be specific, if the UE receiving the HSDPA service enters a cell-overlapped region between an old Node B and a new Node B, then the UE establishes radio links to a plurality of the cells, i.e., a plurality of Node Bs. Here, a set of the cells, to which the radio links are established by the UE, is called an “active set”. In the FCS technique, the UE receives HSDPA service packet data from only the cell maintaining the best channel condition among the cells included in the active set, thereby reducing the overall interference. Here, a cell transmitting the HSDPA service packet data for its best channel condition among the cells in the active set is called a “best cell”. The UE periodically checks the channel conditions with the cells belonging to the active set. Upon detecting a cell having a channel condition better than that of the current best cell, the UE transmits a best cell indicator to all of the cells in the active set in order to replace the current best cell with a new best cell. The best cell indicator includes an identifier of the selected new best cell. Upon receiving the best cell indicator, the cells belonging to the active set analyze the cell identifier included in the received best cell indicator to determine whether the received best cell indicator is destined for them. The selected best cell transmits packet data to the UE using the HS-DSCH.
As mentioned above, in the HSDPA technique, the UE and the Node B are required to exchange the following new control signals in order to use the newly introduced techniques, i.e., the AMC, HARQ and FCS techniques. First, in order to support the AMC technique, the UE informs the Node B of information on the channel between the UE itself and the Node B, and the Node B informs the UE of an MCS level determined according to the channel condition based on the channel information received from the UE. Second, in order to support the n-channel SAW HARQ technique, the UE transmits ACK or NACK (Negative Acknowledgement) signal to the Node B. Third, in order to support the FCS technique, the UE transmits to the Node B a best cell indicator indicating a Node B providing a channel having the best channel condition, i.e., a best cell. In addition, when the best cell is replaced according to the channel condition, the UE informs the Node B of a packet data reception state at that time, and the Node B provides necessary information so the UE can correctly select the best cell.
Information Group #1
Information Group #1 defines information that is transmitted from the Node B to the UE in order to support the HSDPA service. Information Group #1 includes:
1) HSDPA indicator: indicates whether there is HSDPA data for the UE to receive.
2) MCS level information: indicates a transmission technique to be used for the HS-DSCH.
3) HS-DSCH channelization code information: information related to a channelization code to be used for a specific UE by the HS-DSCH.
4) HARQ processor number: indicates a channel transmitting specific packet data among logical channels for the HARQ, when the n-channel SAW HARQ technique is used. Here, the reason for indicating the HARQ processor number is as follows. The n-channel SAW HARQ uses as many HARQ processors as the number n of the channels, so the corresponding packet data is transmitted over a specified one of the channels, and when an error occurs in the transmitted packet data, it must recognize the HARQ processor number corresponding to the errored packet data. That is, the HARQ processor corresponding to the channel number performs a retransmission process on the errored packet data out of the packet data transmitted over the corresponding channel.
5) HARQ packet data number: the Node B informs the UE of a downlink packet data number so that the UE can inform the selected new best cell of a transmission state of the HDSPA data when the best cell is replaced in the FCS technique. Here, the packet data has a SN (sequence number), so it is possible to identify the packet data using the sequence number.
6) UE ID (identification): the Node B indicates whether HS-SCCHs (High-Speed Shared Control Channels) contain control information for the corresponding UE using the UE identification, when part of the above information is transmitted over one or more HS-SCCHs.
In addition to Information Group #1, information that should be transmitted from the Node B to the UE in order to support the HSDPA service includes an uplink transmission power offset value. In some cases, the UE increases the uplink transmission power by an offset so that the adjacent Node Bs belonging to the active set other than the current best cell correctly receive the best cell information transmitted by the UE. In this case, the Node B informs the UE of the uplink transmission offset value. Further, information that is transmitted from the Node B to the UE in addition to Information Group #1 in order to support the HSDPA service includes retransmission information in the HARQ operation, or the number of retransmissions.
The information that is transmitted for the HSDPA service is uniquely provided to the UEs, and this information is transmitted to the UEs before transmission of an HS-DSCH signal that is to be transmitted to a corresponding UE. This is because the information that is transmitted for the HSDPA service is information indispensable for the HSDPA service and the UEs must receive this information in order to normally receive the HSDPA service. Accordingly, there is a need for a method of transmitting the HSDPA service information before transmission of actual packet data. However, the current asynchronous mobile communication system does not have information that is shared by all of the UEs and information on the changed contents that must be simultaneously applied by the UEs and the Node B. like the information used in the HSDPA technique. Thus, there is a demand for a method of transmitting changed information to the Node B and all of the UEs in order for all of the UEs and the Node B to simultaneously equally apply the changed contents in using the HSDPA technique.